Gallium nitride, which is semiconductor material, has become a hot topic of research due to its properties such as a wide band gap, a high electron saturated drift velocity, a high breakdown field strength and good thermal conductivity. Gallium nitride material, compared with silicon or gallium arsenide, is more suitable for fabricating high-temperature, high-frequency, high-voltage and high-power electronic devices. Therefore, the gallium nitride-based electronic device has good application prospects.
Because of strong two-dimensional electron gas in an AlGaN/GaN heterostructure, an AlGaN/GaN HEMT is usually a depletion mode device, and an enhancement mode device is difficult to be achieved. However, applications of the depletion device are limited in many situations; for example, an enhancement mode (normally-off) switching device is required as a power switching device. An enhancement mode gallium nitride switching device is mostly used in high-frequency devices, power switching devices and digital circuits, etc. Therefore, research on the enhancement mode gallium nitride switching device has great significance.
To achieve an enhancement mode gallium nitride switching device, an appropriate method is required to reduce channel carrier concentration under the gate region at zero gate bias, and the methods currently reported include recessing in the gate region, injecting fluorine ions into the barrier layer under the gate, and adopting a thin barrier layer, etc.
Recessing in the gate region is achieved by making slight changes in a structure of a conventional depletion mode AlGaN/GaN HEMT device. A E-beam gate is not directly formed but etching in a pre-deposited gate region is done first and then a Schottky gate is formed in the recessed gate window, where the electron density of electron gas in a channel is reduced by thinning down the barrier layer thickness. To pinch off the channel at zero gate voltage, the thickness of the barrier layer is reduced to less than 5 nm, and in this case, no effective quantum confinement is generated at a positive gate voltage and surface traps are formed, which causes the channel not to be completely opened at a positive gate voltage. In addition, gate leakage current is increased due to electrons in the surface traps. The method of a recessed gate is proposed in 2001 by Kumar et al. in University of Illinois in the United States, referring to Kumar, V., et al., “Recessed 0.25 mm gate AlGaN/GaN HEMTs on SiC with high gate-drain breakdown voltage using ICP-RIE”, Electron. Lett. 2001, 37, pp. 1483-1485.
In case of fluorine implantation, negatively charged ions, such as fluorine ions, are injected into a barrier layer and the two-dimensional electron gas in the conductive channel can be depleted by controlling the dose of the injected ions. A high dose of anions is required to pinch-off the channel. Therefore, a current when the channel is opened is reduced. Y. Cai et al. at Hong Kong University of Science and Technology successfully developed a high performance enhancement mode AlGaN/GaN HEMT in 2005 by using fluoride-based plasma treatment technology, referring to Y. Cai et al., “High-performance enhancement-mode AlGaN/GaN HEMTs using fluoride-based plasma treatment”, IEEE Electron Lett., vol. 2, no. 7, pp. 435-437, 2005.
In case of a thin barrier layer, the electron density of two-dimensional electron gas in a channel is reduced by adapting a thin AlGaN barrier layer. Akira ENDOH et al. at Osaka University in Japan prepare an enhancement-mode device using this method, with a threshold voltage of the prepared enhancement-mode device being zero, referring to Akira ENDOH et al., “Non-Recessed-Gate Enhancement-Mode AlGaN/GaN High Electron Mobility Transistors with High RF Performance”, JJAP, Vol. 43, No. 4B, 2004, pp. 2255-2258.
The methods introduced above all belong to technology of a Schottky gate field-effect transistor and the threshold voltage is generally about 0V-1V, which does not reach the threshold voltage of 3V-5V for application. and the gate leakage current of the Schottcky gate technology is much larger than that of a metal insulator semiconductor field-effect transistor. In addition, plasma treatment is used in both of the method for recessing in the gate region and the method for injecting fluorine ions into a barrier layer under a gate region. However, the plasma treatment will destroy a lattice structure and damage an active region of a device, and repetitive control of the process is poor, therefore, stability and reliability of the device are affected.